6 
THE VOYAGE OF H.M.S. CHALLENGER. 
dimensions, whilst Limacina australis, which was the subject of Souleyet’s investigations^ 
is one of those small forms classed by him under the generic name Spirialis. He 
recognised, however, their close relationship to the larger Limacinse, the only difference 
which he stated to exist between them — the absence of the operculum in Limacina — has 
been found to have no foundation, for it is only that the adults in the large forms have 
lost the operculum. In consequence of the small size of his specimens a number of points 
in their organisation have escaped him. 
In the Challenger collection there was only one specimen of a large Limacina (. Limacina 
antarctica) which I have therefore been compelled to preserve intact ; but since Mr. John 
Murray has placed in my hands a number of specimens of Limacina helicina from Hudson’s 
Strait, I have been able to study a great part of the organisation of the genus, upon a 
large species and upon small forms (Limacina lesueuri and Limacina australis), which 
latter have served more especially for special points and for purposes of comparison. 
Lastly, as regards the genus Perciclis, the form studied is Peraclis reticulata ; but as 
specimens of this genus are very rare, I have only been able to make use of two, and 
hence have not been able to push my researches so far as I could have wished. 
In all the Limacinidse the shell is sinistral, and hence the animal is coiled in a left- 
handed direction ; but although twisted in this manner, in all its organisation the animal 
is dextrorsal, that is to say, that in the asymmetrical disposition the right side pre- 
dominates ; it is here that are found the anus, the genital aperture, and the copulatory 
organ. 
This is a fact opposed to the usual condition in the sinistrorsal Gastropods. In 
Physa, for example, the spiral (and hence the shell) is sinistral ; the anus, the genital 
aperture, and the copulatory organ are all placed on the left side, and hence it is this side 
which predominates in the asymmetry of the animal. Thus the direction of the spiral 
corresponds with the kind of asymmetry observed in this Mollusc. 
The difference between these two cases shows, however, that the mode of asymmetry 
in a Mollusc is in no way dependent upon the direction of its coil. In Physa there is 
a complete situs inversus ; and it is this which has brought about the left-handed 
twisting of the animal and the sinistral character of its shell, for we are acquainted with 
no Gastropod which has acquired a left-handed asymmetry of organisation, and had at 
the same time preserved a dextral shell. In this case, then, the sinistral coiling appears 
to be only one of the consequences of the situs inversus. 
On the contrary, as we see in the Limacinidse, an animal with dextral organisation 
may be coiled sinistrally. The case of the Limacinidse, too, does not seem to be unique, 
since, according to Bouvier , 1 the genus Lanistes (. Ampullaria with left-handed spiral) also 
has a dextral organisation. 
The direction of the spiral, then, does not permit us to determine the mode of thn 
1 Sur le systeme nerveux typique des Prosobranches dextres ou senestres, Comptes rendus, t. ciii. p. 1276, 1886. 
